[SI-LIST] Re: dielectric loss question

  • From: Jim G Roberts <jgroberts@xxxxxxxxxx>
  • To: "chen, jinhua" <chen_jinhua@xxxxxxx>
  • Date: Fri, 14 Feb 2003 19:31:23 +0100

Hi,
        The magnetic field at HF restricts the current paths to the outer
contours. This has two effects.
One as you mentioned is the longer path. This has a two fold effect
it gives dispersion and insertion loss a very degrading effect for data.
Other is that the contour will have many impurities and sharp
ends whic will further increase the loss.
This is corroberated by measurement.
As a microwave engineer in the past it was necessary to
to make fine deposits on the surface of tracks and waveguides to
reduce these losses. This gave a mirror finish.
The highest frequency I worked at was 250 GHz where it was ever
more evident.

Hope this can give you some understanding of the order of things.

Jim

"chen, jinhua" wrote:

> Jim
>
> Could you share the data with me? I did some test recently. The
> skin effect loss is not as bad as I expect at 10 Gbps data rate.
>
> I have two pairs of differential traces with 100 ohms differential
> impedance. One pair is microstrip line with 8 mils line width. The
> second pair is stripline with 5 mils line width. The trace lengths
> are 6" and 12" long for each pair. The eye patterns were took with
> 10 Gbps PRBS data pattern. The difference of eye patterns between
> two pairs is  so small. It let me to believe that the skin effect
> loss is not the significant part at 10 Gbps.
>
> The roughness of copper sufface is two dimentional. One dimention
> increase the length, which increases the resistance. And the other
> dimention increase the perimeter of copper cross section, which
> decreses the resistance. So, they should cancel each other. This
> is just my 2 cents.
>
> Regards!
>
> Jinhua Chen
> Consulting Design Engineer
> 176 South St.
> Hopkinton, MA 01748
> 508-249-4527
> Pager: 8779700234
> chen_jinhua@xxxxxxx
>
> EMC²
> The Enterprise Storage Company
>
> -----Original Message-----
> From: Jim G Roberts [mailto:jgroberts@xxxxxxxxxx]
> Sent: Wednesday, February 12, 2003 9:23 AM
> To: milabont@xxxxxxxxx
> Cc: 'Si-List' (E-mail)
> Subject: [SI-LIST] Re: dielectric loss question
>
> Mike, Peter,
>         It is also interesting that since data rate are in the 10GHz rates
> that
> the
> surface roughness begins to give a large percentage of the losses.
> These are also very frequency sensitive.
> If of interest I have a PPT file.
>
> --
> Regards,    __________   James G Roberts
>            /___  ____ |  jrobert@xxxxxxxxxxxxxxxxxxxxx
>  Jim      __  / /___/ /  jgroberts@xxxxxxxxxx
>          / /_/ /---| |   Room: BE436, Hilversum
>          \____/    /_/   Tel: +31 35 687 4308 Fax: 5976
>
> Mike LaBonte wrote:
>
> > I don't have any info specific to moisture in PCBs. 2.45GHz
> > is where the dielectric permittivity of water begins to decrease
> > significantly, but it is not a peak absorption frequency. Peak
> > dielectric loss for water occurs in the range of 10GHz to 160GHz,
> > depending on the temperature. The peak frequency is higher for
> > higher temperatures. At 25degC the peak is near 40GHz.
> >
> > At DesignCon 2000 Howard Johnson presented "Multi-Level Signaling",
> > in which he gave the rate of change for signal degradation due to
> > skin effect and dielectric loss at 20dB/decade near 1-2GHz. At
> > least half of that is dielectric loss. But the paper also shows
> > noise increasing at 20dB/decade, and SNR falling to zero under 10GHz.
> >
> > My take on it is that water will not be the main cause of signal
> > loss.
> >
> > http://www.sbu.ac.uk/water/microwave.html
> > http://signalintegrity.com/Pubs/misc/mls.htm
> >
> > Mike LaBonte
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Peter Arnold
> > Sent: Tuesday, February 11, 2003 2:35 PM
> > To: 'Si-List' (E-mail)
> > Subject: [SI-LIST] dielectric loss question
> >
> > All,
> >
> > A question from the curious - in high-speed signaling environments we may
> > create time-varying electric fields that have components close to the
> > frequency used to heat water in microwave ovens, somewhere around 2.45GHz.
> > This suggests a portion of dielectric loss at these frequencies might be
> due
> > to dielectric heating of any moisture that might be present in the
> > fiberglass.
> >
> > Is this in fact the case, and what magnitude is the effect at normal
> > humidities? Does loss increase in real multi-GHZ signaling systems as
> > dielectric moisture content increases?
> >
> > Thank you,
> > peter arnold.
> >
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--
Regards,    __________   James G Roberts
           /___  ____ |  jrobert@xxxxxxxxxxxxxxxxxxxxx
 Jim      __  / /___/ /  jgroberts@xxxxxxxxxx
         / /_/ /---| |   Room: BE436, Hilversum
         \____/    /_/   Tel: +31 35 687 4308 Fax: 5976



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